a. Field of the Invention
This invention relates to unions or connections between objects made from different materials.
b. Background of the Invention
There are many reasons to connect dissimilar materials. Every material has its own set of characteristics, and the characteristics often have certain advantages and disadvantages for a particular service. For example, polymers are typically light weight and flexible, but many metals are very strong and ridged. Often times a component will have different requirements at different locations, so the parameters for selecting a material will vary from one location to another. Many times, a designer will want a strong, secure connection between two dissimilar materials. This can facilitate a change of materials for different locations while maintaining the integrity of a component or part.
For example, a pipe may use one material that resists corrosion in a corrosive area, and the same pipe may use a lower cost material in less corrosive areas. There should be a strong union between the two different materials so the contents of the pipe don't leak or spill. An antenna or rod may use a very strong material at a stress point, and a light weight material for areas where the antenna extends upward to minimize weight, and a union between the strong and light weight material is necessary to support the upward extension. An electrical line may use a flexible, highly conductive material in a location where the line passes through a conduit, and a lower cost material could be used to cover long distances where the line is suspended from poles. A good connection between the two materials is necessary for electrical conduction. There are many other examples where a strong, durable union between dissimilar materials is desirable.
Many techniques can be used for joining dissimilar materials. Perhaps the most obvious is a mechanical connection, such as using threads to screw two pieces together, or the use of nails or rivets to hold parts together. A friction fit, also referred to as an interference fit, involves placing parts tightly together such that friction holds them in place. Factors that affect a friction fit include the union surface area, the number of surfaces involved, surface materials and surface texture. There are many other types of mechanical connections, and as with different materials, different types of connections have various characteristics with advantages and disadvantages for specific applications. Some mechanical connections can work loose, and many mechanical connections will not form an air-tight seal.
Another type of connection uses a binder of some sort. A welded joint involves actual melting and mixing of the components, and a filler material is often used. A welded joint is typically very strong and airtight. Welding may require high localized heat that can distort parts, it often requires skill to apply, and the union may require additional clean-up work. Polymeric parts can be welded together without a filler material, but polymeric welding is primarily used with thermoplastic polymers as opposed to thermoset polymers. A solvent can be used with PVC parts to partially dissolve and effectively “melt” different pieces together.
Brazing or soldering melts a filler material but not the component parts. Brazing is very similar to soldering, except higher temperatures are used. The component parts are placed close together with small tolerances, and the filler material is melted and flows by capillary action between the parts. The union is then cooled so the filler material solidifies and holds the parts together. Brazing and soldering make a strong connection that is airtight, and the union is typically clean so little or no additional work is required after formation. Alternatively, different components parts can be placed together with an adhesive between them, and the adhesive attaches to each part. Many adhesives do not require any heating, but the components often need to be held together without disturbance for a period of time to allow the adhesive to “set”. Adhesives can make air-tight unions, but the connection is typically not as strong as a brazed or soldered part.
It can be more difficult to form a strong, durable union between dissimilar materials than between similar materials. For example, many dissimilar metals melt at different temperatures, as well as having different thermal conductivities and coefficients of thermal expansion, so many dissimilar metals are difficult to weld together. Many dissimilar materials can be brazed together, but the connection is often weaker than when brazing similar materials. A brazing compound can be selected that is very effective for one material, but the same brazing compound may not be as effective for the other material. Therefore, a brazed connection between two dissimilar materials may not be as strong as a brazed connection between similar materials. The same principle applies to soldering, and may apply to adhesives as well.
One potential disadvantage of connecting dissimilar materials is galvanic corrosion, especially when working with metals. Galvanic corrosion is well documented for metals, and can include materials such as graphite. Galvanic corrosion is sometimes referred to as electrolysis or dissimilar metal corrosion. When two different metals that are in electrical contact are placed in a common electrolyte, a current is produced that can cause one metal to discharge ions into the electrolyte. This reduces the amount of metal at the union, and serves to accelerate corrosion. Distilled water is not an electrolyte, but water readily absorbs or dissolves small amounts of various compounds, and this makes the water an electrolyte. Most water found in nature has dissolved compounds and is an electrolyte, and water is the most common electrolyte involved with galvanic corrosion (but other electrolytes are possible). Water found in coastal areas or around salt water can contain higher concentrations of compounds, and makes a very strong electrolyte that can accelerate corrosion.
Galvanic corrosion is more rapid when the difference in the electrical potential of the metals involved is large. The higher the difference in electrical potential (also referred to as galvanic potential or electrode potential), the more rapid the corrosion. The less noble metal discharges ions into the electrolyte in galvanic corrosion, so the less noble metal degrades more rapidly than the more noble metal. The higher the electrical potential, the more noble the metal. The more noble metal will act as a cathode, the less noble metal will act as an anode, and the dissimilar metals will effectively form a battery that drives the galvanic corrosion. Some techniques that can be used to combat galvanic corrosion include (i) use metals with a small difference in electrical potential, (ii) prevent simultaneous contact of the two metals with an electrolyte, and (iii) electrically isolate the two metals, such as by physically separating them or placing an insulator between them.
A sacrificial anode can be used to help combat galvanic corrosion. A sacrificial anode will typically be placed in electrical contact with the part to be protected, and immersed or exposed to the same electrolyte. The sacrificial anode should have a lower electrical potential than the part being protected, so the sacrificial anode is less noble. The sacrificial anode will discharge ions more rapidly than the more noble part being protected, so the more noble part will not erode until the sacrificial anode either decomposes, or no longer contacts the electrolyte, or loses its electrical connection with the item being protected. Other factors can also be involved, such as distance and surface area.